Under certain operating conditions, engines that have high compression ratios, or are boosted to increase specific output, may be prone to low speed pre-ignition combustion events. The early combustion due to pre-ignition can cause very high in-cylinder pressures, and can result in combustion pressure waves similar to combustion knock, but with larger intensity. Strategies have been developed for prediction and/or early detection of pre-ignition based on engine operating conditions. Additionally, following detection, various pre-ignition mitigating steps may be taken.
The inventors herein have recognized that not all cylinder pre-ignition events are the same, and that pre-ignition mitigation steps may need to be adjusted based on the nature of the pre-ignition event, as well as the pre-ignition history of the cylinder. For example, mitigating steps used for a cylinder with sporadic pre-ignition may not be as effective for a cylinder with recurrent pre-ignition. In other words, a more aggressive approach to pre-ignition mitigation may be required during some pre-ignition events as compared to other pre-ignition events.
Thus, in one example, the issue may be addressed by a method of operating an engine comprising, in response to intermittent pre-ignition in a cylinder, enriching the cylinder, and limiting engine load by a first amount, and in response to persistent pre-ignition in the cylinder, enriching the cylinder, and limiting engine load by a second amount, greater than the first amount.
In one example, in response to an indication of cylinder pre-ignition, an engine controller may update a cylinder pre-ignition history. The indication of pre-ignition may be based on engine operating conditions such as knock intensity (as determined by a knock sensor), a crankshaft acceleration (as determined by a crankshaft sensor), spark plus ionization, and/or based on changes in cylinder pressure as a result of abnormal cylinder combustion events. The cylinder pre-ignition history may include, for example, a cylinder pre-ignition count, including a number of cylinder pre-ignition events that have occurred over the lifetime of the cylinder's operation, as well as a number of cylinder pre-ignition events that have occurred over the current engine cycle. The history may further include a number of consecutive pre-ignition events in the cylinder. In one example, in response to a plurality of discrete pre-ignition events over a plurality of consecutive cylinder combustion events (that is, a number of consecutive cylinder pre-ignition events being less than a threshold), an engine controller may determine intermittent pre-ignition in the cylinder, and may adjust the mitigating steps accordingly. In another example, in response to a plurality of continuous pre-ignition events over the plurality of consecutive cylinder combustion events (that is, a number of consecutive cylinder pre-ignition events exceeding the threshold), the engine controller may determine persistent pre-ignition in the cylinder, and may adjust the mitigating steps accordingly.
For example, in response to the indication of intermittent pre-ignition in the cylinder, the controller may immediately enrich the cylinder and limit the engine load by a first amount, while in response to the indication of persistent pre-ignition in the cylinder, the controller may immediately enrich the cylinder by a second, larger amount. In both cases, enriching the cylinder may include operating the cylinder at an air-to-fuel ratio richer than stoichiometry for a given duration. By enriching the cylinder in response to an occurrence of pre-ignition, an immediate cylinder air charge cooling effect may be achieved that may reduce the occurrence of further abnormal combustion events. The simultaneous limiting of engine load, for example by reducing air flow, can further assist in reducing the occurrence of additional pre-ignition events. However, the effect of load limiting on pre-ignition may be delayed until a stable air flow is reached.
The enrichment in response to the intermittent pre-ignition may differ from the enrichment in response to the persistent pre-ignition. For example, the enrichment in response to the intermittent pre-ignition may be less rich and/or for a shorter duration while the enrichment is response to the persistent pre-ignition may be more rich and/or for a longer duration. The engine load limiting may be similarly adjusted. As such, limiting an engine load may include reducing the air flow by adjusting one or more of a throttle opening, engine boost, cam timing, valve timing, waste-gate timing, etc. Thus, in one example, in response to the intermittent pre-ignition, throttle opening and boost may be reduced by a smaller amount, while in response to the persistent pre-ignition, throttle opening and boost may be reduced by a larger amount. In another example, the camshaft timing may be adjusted by a larger amount in response to persistent pre-ignition and by a smaller amount in response to intermittent pre-ignition.
In one example, the load limiting may be synchronized with the enrichment by performing the load limiting at a ramp-in rate that is coordinated with the enrichment operation. For example, the ramp-in rate may be adjusted such that ramping in of the limited load is completed concurrent to completion of the enrichment.
In another example, an engine may include a first and second group of cylinders. Herein, in response to intermittent pre-ignition in a cylinder in the first group of cylinders, the controller may enrich the given cylinder and limit an engine load of all cylinders in the first group, and not the second group, for example by adjusting a cam timing of first group but not the second group. In comparison, in response to persistent pre-ignition in a cylinder in the first group, the controller may enrich the given cylinder and limit an engine load of all cylinders in the first and second group. In one example, this may include limiting an engine load of all cylinders in the first and second group by the same (larger) amount. In another example, this may include limiting an engine load of all cylinders in the first group by a larger amount while limiting an engine load of all cylinders in the second group by a smaller amount.
Similarly, the enrichment profile for other cylinders of the different groups may be adjusted differently based on the nature of the pre-ignition. For example, in response to persistent pre-ignition in a cylinder in the first group, the given cylinder may be enriched more rich and/or for a longer duration, while other cylinders in the first group are also enriched, but for a shorter duration and/or less rich. In another example, cylinders in the second group may also be enriched in response to the persistent pre-ignition to reduce the likelihood of further engine pre-ignition events. In comparison, in response to intermittent pre-ignition in a cylinder in the first group, only the given cylinder may be enriched. Still other combinations may be possible.
Further still, in addition to the enrichment and load limiting, cylinder spark timing may be advanced by an amount. Specifically, spark may be advanced, relative to the spark timing at the time of pre-ignition detection, towards MBT. The amount of spark advance may be adjusted based on the current engine speed, the enrichment, and/or the nature of the pre-ignition. Thus, as the degree of richness and/or duration of enrichment increases, the amount of spark advance may be increased. Further, a higher amount of spark advance may be used in response to persistent pre-ignition while a smaller amount of spark advance may be used in response to intermittent pre-ignition. Since the cylinder may be more tolerant to spark advance due to the richer than stoichiometry air-to-fuel ratio during the enrichment, spark advance may be advantageously used in conjunction with the enrichment to maintain IMEP under the rich conditions of the cylinder.
The updated pre-ignition history (including the updated pre-ignition count) may also be used to determine a likelihood of pre-ignition in a cylinder. For example, at the onset of engine operation, a controller may determine a feed-forward likelihood of pre-ignition based on the engine's operating conditions and further based on the pre-ignition history of the engine's cylinders, and accordingly may preemptively limit an engine load based on the likelihood of pre-ignition. The above described pre-ignition mitigating steps (enrichment and further load limiting) may then be applied in response to the indication of pre-ignition, and the nature of the pre-ignition.
In this way, by adjusting pre-ignition mitigating steps based on the nature of the pre-ignition, pre-ignition related issues may be better addressed, and further occurrences of pre-ignition may be reduced. Specifically, by responding more aggressively to persistent pre-ignition and less aggressively to intermittent pre-ignition, engine pre-ignition may be better addressed.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.